1. Field of the Invention
This invention relates to a technology of suppressing a housing temperature in an electronic device.
2. Description of the Related Art
In a portable computer such as a notebook personal computer (PC) or a personal digital assistant (PDA), performance of a semiconductor device, such as a central processing unit (CPU), a video chip, or a CPU bridge has been improved, resulting in increased heat generation. On the other hand, while the notebook PC is typically reduced in thickness or size, a packaging density of the semiconductor device in a housing is increased. Since an allowable temperature during an operation is specified for each semiconductor device, a cooling device, such as a cooling fan or a heat sink is provided in the notebook PC for cooling to maintain an allowable temperature.
FIG. 8 is a cross-sectional view showing a cooling device in a conventional notebook PC and a flow of air around this device. The cooling device is formed of a heat sink 359, a heat pipe 353, and a cooling fan 357 accommodated in a housing 311. The heat pipe 353 transmits heat from a semiconductor device having a large amount of heat generation, such as a main CPU or a video chip to the heat sink 359. The cooling fan 357 takes in air in the housing from an upper side and a lower side as indicated by an arrow A′ and an arrow B′, and discharges air from an exhaust opening 327 through the inside of the heat sink 359 as indicated by an arrow C′. The heat sink 359 diffuses heat transmitted through the heat pipe 353 into air passing through the inside.
A periphery of the exhaust opening 327 is hermetically sealed by a sealing material 371 to prevent a high-temperature air which has passed through the heat sink 359 from flowing back into the housing 311. Inlet openings 367 and 368 are provided at respective positions in a top cover 381 and a base cover 321. Since a part of the housing 311 except the inlet openings 367 and 368 are substantially hermetically sealed, outside air flows in from the inlet openings 367 and 368 to produce air currents indicated by arrows D′ and E′ when the cooling fan 357 operates. The air currents cool each semiconductor device on a circuit board 341 or other devices in the housing 311.
The cooling device shown in FIG. 8 has a structure where a negative pressure is formed in the housing 311 to take in outside air from the inlet openings 367 and 368 provided at the respective positions and the air is discharged to the outside of the housing through the heat sink. Therefore, flow paths or flow rates of the air currents D′ and E′ must be determined to allow effectively cooling components in the housing. The flow paths or the flow rates are determined to obtain an optimum cooling effect by repeating an experiment in which mainly positions and sizes of the inlet openings 367 and 368 are used as parameters.
Further, structural considerations may prevent the inlet opening 377a from being placed near the side surface like FIG. 9A, and so the inlet opening 377a is actually placed at a position on the cooling fan 357 side as depicted in FIG. 9B. In this case, since an air current indicated by an arrow H′ flows, a volume of a heated air layer 375b is increased, and reducing a temperature of the housing becomes further difficult.
FIG. 9C shows an example where another inlet opening 377c is provided in a side surface of a base cover 321c. In this case, since an air current passes under the heat sink 359 as indicated by an arrow I′, a heated air layer is not accumulated below the heat sink 359. However, high-temperature air immediately after discharge from the exhaust opening 327 is mixed into outside air taken in from the inlet opening 377c as indicated by an arrow J′, and hence a temperature of an air current I′ is increased, and a temperature of the base cover 321 cannot be reduced. Furthermore, in all of the examples depicted in FIGS. 9A to 9C, when the inlet openings 377a to 377c are newly provided near the cooling fan 357, flow paths or flow rates of the air currents flowing through other parts vary, and hence an overall air balance in the housing must be reexamined.